PROCEEDINGS OF THE YEREVAN STATE UNIVERSITY
Chemistry and Biology
2014, № 3, p. 3–7
Chemistry
A STUDY OF THE INTERACTION BETWEEN RHENIUM(VII)
AND ALLYLTHIOUREA BY SPECTROPHOTOMETRY
H. H. DARBINYAN, H. G. KHACHATRYAN
Chair of Inorganic and Analytical Chemistry YSU, Armenia
The possibility to obtain a complex compound of the rhenium(V) with
allylthiourea has been studied. The optimal conditions, in which the chemical
reaction took place, have been estimated. It has been shown than this reaction may
be used for spectrophotometric determination of rhenium(VII).
It has been approved by means of potentiometric titration method that 1 mol
quantity of tin(II) chloride is necessary to reduce 1 mol quantity of perrhenate,
which witnesses about the reducing of the Re(VII) to Re(V). Allylthiourea and
Re(V) form a new yellow colored complex substance having maximal light
absorption at λ = 365 nm wavelength.
Keywords: perrhenate, allylthiourea; rhenium(V)-allylthiourea complexes.
Introduction. The questions relating to the study of the chemical-analytical
properties of rhenium and its quantitative determination were discussed in detail in
some fundamental sources [1–3], where voluminous information was given on the
chemical state of the rhenium in aqueous solutions, methods for its isolation and
determination, as well as information connected to the methods for analysis of the
rhenium-containing materials (alloys, ores, etc.).
The peculiarities of the rhenium chemistry in the solutions are connected
with its ability to realize variety of oxidation states in its compounds: rhenium
compounds may undergo hydrolysis, oxidation-reducing (particularly disproportionation). The most stable in aqueous solutions among them are Re(VII) and
Re(IV) compounds.
One of the characteristic properties of the rhenium in lower oxidation states
is its inclination to the complex formation inherent to all transition metals. When
studying complex formation reactions the particular attention is spared to the search
of optimal conditions for forming colored compounds, their stability, their composition
and structure. When acting with Re(V) the thiourea derivatives form yellow-colored
complex compounds having maximal light absorption at λ = 365 nm wavelength [3].
The synthesis of Re(V) complexes with several organic sulfur-containing
ligands has been described later in [4–7]. A possibility of obtaining the complex of
Re(V) with allylthiourea has been studied by us in the recent work.

E-mail: [email protected]
Proc. of the Yerevan State Univ. Chemistry and Biology, 2014, № 3, p. 3–7.
4
Experimental Part. Perrhenate spare solution with 1·10–2 mol/L concentration has been prepared by dissolution of the accurate weighed sample of KReO4 in
distilled water. The allylthiourea (ATU) solution with 1·10–2 mol/L concentration
has been prepared by dissolution of the accurate weighed recrystallized preparation
in distilled water. 10% solution of tin(II) chloride in diluted hydrochloric acid (1:1)
has been used as a reducing agent, and the acidity of the reaction medium has been
approved by addition proper volume of concentrated hydrochloric acid.
The method of potentiometric titration of perrhenate by standard solution of
Sn(II) chloride has been applied for determination of oxidation state of rhenium in
the experiment conditions (2.0–5.0 mol/L acidity by hydrochloric acid). Platinum
has been used as an indicative electrode. The system potential was settled slowly
and the titration jump was observed after addition of the accurately 1 mol reducing
agent. It means that in the above mentioned conditions Re(VII) is reduced to Re(V)
oxidation state (Fig. 1).
The Re(VII)– Sn(II)– ATU system has been studied by spectrophotometric
method as well to confirm oxidation state of rhenium in the obtained compound
(the colored compound has maximal absorption at 365 nm wavelength). For this
purpose isomolar series of solutions has been prepared, where ATU concentration
has been kept constant (surplus) and concentrations of Re(VII) and Sn(II) chloride
was varied (the sum of concentrations was kept 5.0·10–3 mol/L). The maximal light
absorption was observed at Re(VII) : Sn(II) = 1 : 1 molar ratio, which corresponds
to the reducing of Re(VII) to Re(V) (Fig. 2).
E, mV
Optical Density, A
n perrhenate/ n allylthiourea
Fig. 1. Potentiometric titration of perrhenate by
Sn(II) chloride:
[ReO4–] = 5·10–3 mol/L;
[ATU] = 5·10–3 mol/L.
Vperrhenate, mL
Fig. 2. Isomolar series of the Re(VII)
and Sn(II) solutions (total concentration
5·10–3 mol/L) in the presence of ATU:
VSn(II) chloride + Vperrhenate = 10.0 mL.
The study of the interaction of ATU with perrhenate in the presence and in
absent of the reducing agent has been shown that the addition of the great surplus
of the reagent to the potassium perrhenate acidic solutions leads to the formation of
the yellow-colored complex compound. The maximal absorption is observed at the
same wavelength 365 nm.
Darbinyan H. H., Khachatryan H. G. A Study of the Interaction Between Rhenium (VII)…
5
Inasmuch ATU possesses reducing properties one can assume that Re(VII)
participates to the complex formation in lower oxidation state. The reaction proceeds
slowly and it is expedient to introduce off-site reducing agent – Sn(II) chloride.
The saturation method has been used when studying the influence of tin(II)
chloride concentration on the complex formation of rhenium with ATU. The solutions
investigated contained constant concentration of rhenium(VII) (5.010–3mol/L),
ATU (5.010–3mol/L) and variable concentration of tin(II) chloride. Acidity
corresponds to 3.0 mol/L by hydrochloric acid and the volume of the solutions was
kept constant (25 mL).
It has been ascertained that 20-fold surplus of the Sn(II) chloride is enough
to assure complex formation of Re(VII) with ATU. Higher content of the reducing
agent did not change optical density values of the system. The invariable maximal
values had reached within 15–20 min and then stay invariable during 24 h and even
longer. Moderate heating of the system (up to 70ºC) promotes faster development
of the color (5–7 min).
The order of the reagents addition also is important. The maximal coloring
was secured in case when first requested amount of the hydrochloric acid was added
to the perrhenate solution, then allylthiourea solution, and finally – the tin(II)
chloride solution.
The colored Re(V)–ATU compound forms in hydrochloric acid media.
Absorption spectra obtained in the presence of different concentration of hydrochloric acid showed that the maximal optical density for the forming compound is
observed when acidity corresponds to 2.0–6.0 mol/L by hydrochloric acid).
The saturation method (the method of continuous change of the reagent
concentration) has been applied to estimate the required amount of ATU keeping
the rhenium(VII) concentration constant (1.010-4mol/L and 2.010–4mol/L). The
maximal coloring of the complex is developed already at three-fold molar surplus
of the ATU. Further increase of the reagent concentration has no influence on the
optical density values and it may witnesses that only a complex with one
composition arose.
Optical Density, A
Vperrhenate, mL
Fig. 3. Isomolar series of the solutions with total concentration rhenium(VII) and allylthiourea
equals to 4 10–4 mol/L (1) and 8 10–4 mol/L (acidity – 3.0 mol/L by hydrochloric acid);
Vperrhenate + V allyithiourea = 10.0 mL.
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Proc. of the Yerevan State Univ. Chemistry and Biology, 2014, № 3, p. 3–7.
The isomolar series method indicates that molar ratio of the acting reagents
in the forming compounds is as following [Re(VII)] : [ATU] = 1 : 3 (Fig. 3).
Calibration curve is linear for the Re(VII) contents within 0.8–8.0 mcg/mL
range.
The molar extinction coefficient () calculated according to the calibration
curve data equals 3550 cm·L·mol–1 at 365 nm wavelength. Insufficiently sharp
maximum at the isomolar series curves may witnesses about partial dissociation of
the obtained complex compound. We assume that the reaction proceeds according
to the following Scheme:
AllylHN
NH2
AllylHN
S
NH
SH
ReO4- + 4H+ + Sn2+
ReO2+ + 2H2O + Sn4+
_
NH
NH
ReO2 (AllylHN
ReO2+ + 2AllylHN
SH
)
S
2
+ 2H+
When studying the interfering influence of several elements during the
spectrophotometric measurements of the optical density of Re(V)–ATU complex it
has been revealed that molybdenum and tungsten may form similar complexes.
That is these elements will interfere rhenium(VII) determination causing
overestimated results. Under stated results have been obtained in the presence of
mercury(II), bismuth(III). Selenium(IV) may be reduced to the elementary state
(red precipitate), tellurium(IV) form yellow-colored compound. So, all mentioned
elements interfere rhenium(VII) determination by ATU.
Conclusion. The presence of tin(II) chloride rhenium(VII) interact with
ATU forming yellow-colored complex compound with maximal light absorption at
λ = 365 nm wavelength. The optimal conditions for arising the complex compound
have been revealed: 20-fold surplus amounts of the reducing agent (SnCl2).
At least three-fold surplus of the reagent (ATU); medium acidity is
0.5–5.0 mol/L by hydrochloric acid; color development time is 20 min; color is
stable more than 24 h.
The molar ratio between reacting agents is following:
[Re(VII)] : [ATU] = 1 : 3.
This fact has been confirmed by means of saturation and isomolar series
methods.
Received 17.06.2014
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